Astrocytic regulation of neuronal synchronization. The function that astrocytic glia play is poorly understood, and their traditional role, as supportive of neurons, captures only a small part of many functional outputs that astrocytes may have. Recent research on astrocytes has revealed important effects on synaptic transmission, but their role in the circuit as a whole has been less studied, partly due to lack of circuit-level assays of the interaction between astrocytes and neurons. Using two-photon imaging, we have recently discovered that, in neocortical brain slices, stimulation of a single astrocyte can lead t widespread neuronal synchronization, in the form of UP states. Conversely, blocking astrocytic signaling by the injection of BAPTA into individual astrocytes can reduce the number of spontaneous neuronal UP states. We propose to test in a series of direct experiments if astrocytes regulate UP states in vivo. We have developed new two-photon and genetic techniques that enable us to image and manipulate the activity of astrocytes in vivo and examine how it relates to neuronal UP states. Our central hypothesis is that the activity of astrocytes precedes, and is causally related, to the occurrence of UP states. Specifically, in a first aim we will perform fast two-photon calcium imaging, with SLMs, of neuronal and astrocytic activity during UP states in Brainbow mice where astrocytic territories are labeled, and examine whether there are spatio- temporal correlations between the activity of astrocytes and neurons. In a second aim we will optically stimulate astrocytes, using two-photon uncaging of RuBi- glutamate, IP3 or ChR2 photoactivation, and test whether this increases or alters spontaneous UP states. Our proposed work could establish a causal link between astrocytic function and neuronal synchronization in vivo, thus providing a novel circuit-level functional role for astrocytes. UP states are thought to underlie slow-wave sleep and resting state fMRI signals, so our research could directly involve astrocytes in the circuit mechanisms of those global brain states. Our group has a unique combination of optical and circuit neuroscience skills and this research could introduce novel approaches into the study of astroglia.